U.S. Pat. No. 4,623,503 to Anestis et al. entitled “Slush Molding Method With Selective Heating of Mold By Air Jets”, assigned to the assignee of the present invention and hereby incorporated by reference, discloses a method of slush molding with the use of an electroformed nickel mold.
According to U.S. Pat. No. 4,108,740 to Wearmouth entitled “Hard, Heat-Resistant Nickel Electrodeposits”, the production of electroforms involves building up deposits of adequate thickness on a mandrel without internal stress in the deposit so high as to cause premature separation of the deposit from the mandrel. The '740 patent goes on to state that the electroformability and hardness of nickel can be improved by electrodepositing the nickel from an electrolyte containing addition agents which introduce sulfur into the resulting electrodeposit and that, while sulfur improves electroformability by reducing the internal stress in the electrodeposit, it does so at the expense of ductility. In the '740 patent, for example, it is reported that sulfur contents in excess of approximately 0.005% cause the electrodeposit to embrittle upon exposure to temperatures above about 200 degrees Celsius, and that embrittlement at temperatures above ambient is particularly disadvantageous in electroforms requiring exposure to elevated temperatures, in applications such as molds and dies, or in fabrication such as screen printing cylinders which can be subjected to localized heating by brazing, welding or by the use of heat curable glues, or during surface masking using heat curable lacquers.
According to U.S. Pat. No. 5,470,651 to Milinkovic et al. entitled “Mandrel For Use in Nickel Vapor Deposition Processes And Nickel Molds Made Therefrom” one drawback of electroformed nickel shells and molds, in consequence of the fact that electroformed nickel contains relatively large amounts of sulpher, is that repairs or modifications to the shell or mold by means of welding cannot be preformed readily.
In addition to the above drawbacks, the Applicant has found that electrodeposition structures, such as the electroformed molds discussed above, may contain voids within the electrodeposition structure itself. These voids are formed during the build-up of deposits on the mandrel and are ordinarily of microscopic size, generally round in shape and on the magnitude of less than 0.005″ in size.
Applicant has also found that, during heating of the electrodeposition structure, these voids, depending on their proximity to the surface of the electrodeposition structure, may cause the surface of the electrodeposition structure to distort in the form of a protuberance, similar to that of a bulge or bump, on the electrodeposition surface. Without being bound to a particular theory, the Applicant believes that heating of the electrodeposition structure causes pressure from gas, believed to comprise hydrogen generated and entrained during formation of the electrodeposition structure, within the void to increase. As a result, particularly in those areas of the electrodeposition structure where the voids are nearest the surface, the increase in gas pressure within the void overcomes the bending strength of the thin electrodeposition thickness above the void and forces the surface of the electrodeposition structure to rise.
In those instances where the voids produce surface protuberances, the Applicant has found that the voids may be repaired via welding. However, more problematic is whether the texture of the surface of the weld and surrounding electrodeposition structure are uniform and blended as to completely hide the presence of the repair. Applicant has found that the ability to repair the surface of the weld and surrounding electrodeposition structure adequately depends largely on the texture of the surface of the electrodeposition structure. Many of the electroformed molds used in the automotive industry have a grain texture formed on the electrodeposition surface. In some instances the texture of the electrodeposition surface can be repaired, while in other instances it cannot be successfully repaired as the grain pattern cannot be replicated in the repaired area. Thus, at the very least, voids in the electrodeposition structure result in costly repairs and time and, on occasion, the complete electrodeposition structure becomes scrap.
Furthermore, Applicant believes that while certain of the voids contained within the electrodeposition structure may not produce protuberances on the surface of the electrodeposition structure in response to heating of the structure, nevertheless Applicant believes these voids may weaken the overall electrodeposition structure resulting in premature cracks, metal fatigue, etc. of the electrodeposition structure.
What is needed is a process to anneal an electrodeposition structure to make the structure more ductile so as make the structure more receptive to repairs or modifications by means of welding. What is also needed is a process to anneal the electrodeposition structure such that the likelihood of voids which may be formed in the structure, giving rise to protuberances on the surface of the structure during heating, is reduced and more preferably eliminated.